The row over Heathrow has escalated further as London Mayor Boris Johnson called on David Cameron to rule out a third runway beyond the next general election.

Following the removal of Justine Greening as Transport Secretary in Tuesday's Cabinet reshuffle, Mr Johnson said it was time for the Prime Minister to "end the anxiety" over the airport by making clear that the Government's pledge not to approve a third runway would not be ditched after the 2015 poll.

New Conservative chairman Grant Shapps took to the airwaves to insist that there remained an "absolute guarantee" of no expansion at Heathrow during this Parliament, but Tory MP and environmental campaigner Zac Goldsmith said that Ms Greening's move made clear that the Government was "opening the door" to the possibility of a third runway.

Meanwhile, Deputy Prime Minister Nick Clegg dismissed talk of a rightwards shift in the reshuffle, insisting the coalition Government "remains anchored in the centre ground".

Speaking during a visit to an east London school a few hours before the first meeting of the new Cabinet, Mr Clegg said: "Right from day one this Government was anchored in the centre ground. We've got a coalition agreement which is there, which is a tablet of stone setting out what we are going to do. That is not going to change."

Liberal Democrats are privately dismayed by changes in Mr Cameron's team of Tory ministers, such as the replacement of the liberal Kenneth Clarke as Justice Secretary by hardliner Chris Grayling.

Lib Dem president Tim Farron said the Prime Minister's apparent swing to the right was proof that Mr Cameron's project to modernise the Tories had failed and his claim to the centre ground had "evaporated".

"If we have got Conservative ministers who want to do more right-wing, reactionary things, it shows the Liberal Democrats are all the more important in stopping them," said the Westmorland and Lonsdale MP.